While I use Dropbox for my file sharing and synching, However, I also use Droplr as a part of my workflow.
One thing Dropbox doesn't do is screenshots. When I am working on a client design, I will often screenshot the progress, just to give the client a quick visual update.
Using my custom hotkey, it's a snap to screenshot, and upload, and be immediately given a URL to paste into my client emails. I don't really focus on "versions" of my work (for that, I'd use Adobe Creative Cloud) ... so I don't usually ever go back to a Droplr upload. I screencap it, send it and forget it. And it just works.
I also love that premium users can use a custom domain. Everything I send to clients appears as if it's coming from my main website, displayed and showcased nicely.
That said, I do wish they'd give an option for annotations. I don't use them myself, and it's an extra click for me to "upload". This is a newer feature, and I honestly preferred the faster/auto upload.